Tide corrections from KGPS and a precise geoid

1. Tide corrections from KGPS and a precise geoid John Brozena – Naval Research Lab john.brozena@nrl.navy.mil

2. Goals • Approximate tide datum from a precise gravimetric geoid and an offset determined from one or more tide gauges or GPS buoys • Determine hydrographic tide correction from KGPS vertical shipboard positions referenced to geoid + offset

3. Connecting the Tide Datum to the Ellipsoid • Global geoid is the gravitational equipotential surface that approximates MSL on a global basis. • Local MSL deviates from the global geoid due to oceanographic and atmospheric effects • mean currents • water column density anomalies • non-standard average air pressure • wind set-up • tidal nodes • Tidal datum is defined locally as a long-term average of low tides below local MSL (tide gauge). • Approximate tide datum by a geoid with a constant or slowly varying offset.

4. Tampa Bay MSL-Datum Offset (Parker et al., 2001) 39 cm MSL-MLL offset produces a max 5 cm datum error

5. meters Local NRL Geoid

6. meters Local Residual Geoid (EGM96- NRL Local Geoid) Shows high frequency information not contained in EGM96 field

7. Goddard MSS Model – NRL Local Geoid Tide Gage MSS-geoid MSL-geoid MSL-MLW pcb 0.11 0.04 0.20 pensb 0.04 0.04 0.19 wavms 0.10 0.16 0.24

8. Ship as Tide Gauge • “Reducing” the GPS position to the water level yields an instantaneous measurement of water surface with respect to the geoid • If the local offset between the ellipsoid and the tidal datum can be determined, the ship is essentially a continuous free-floating tide gauge

9. Roll Ship-Antenna Geometry: Changes in Attitude • GPS solution locates the antenna atop the boat • Need to relate antenna position to sea surface (or need instantaneous keel depth) • Vertical distance changes with static draft, dynamic draft, and boat-antenna lever-arm geometry Squat

10. NRL/NAVO Bertram Experiment • Three day survey in May 2002 • Conducted aboard the Bertram from Gulfport tide gauge • Sea surface height plus ship attitude information measured while repeatedly traveling between four tide gauges: Gulfport, Ship Island, Waveland, and Point Cadet. • The first three of these have been referenced to the ellipsoid by static GPS surveys.

11. Bertram Instrumentation • Ashtech Z12 dual-frequency GPS receiver • GPS antenna mounted on mast above bridge • Inertial navigation system and tilt meter for attitude information • Gulfport tide gauge was set up as the GPS base station

12. Bertram Gulfport Survey • Each day began at the Gulfport tide gauge with half hour GPS collection at port • Bertram traveled to each of the other three tide gauges repeatedly, returning to the Gulfport gauge in between • Half hour occupations at the other tide gauges were also performed several times daily to help constrain biases associated with antenna-boat geometry, the geoid, and the links into each gauge.

14. Sea Surface Height for Day 129 GPS Reference Red=local geoid Blue=EGM96

15. Sea Surface Height with 200 s Gaussian Filter

16. Squat Correction- 1st Iteration

17. Final (speed-based) Squat Correction

18. Squat and Tide Adjusted SSH RMS residual = 4.0 cm

19. RMS residual = 4.1 cm

20. Chesapeake Bay Airborne Survey • P-3 airborne sea-surface height measurement over 8 tide gauges in the bay • SSH calculated using GPS heights and a radar altimeter • Geoid 99 is a relatively good geoid for the region but high frequency information is lacking • 3 cm rms accuracy for survey

21. Chesapeake Bay Airborne Survey

22. Summary • Current project completed • Method works extremely well for this case • Good gravity coverage/geoid • Constant datum offset assumption valid for region • Next step: Demonstrate how the sea surface as measured with GPS can be related to the tidal datum using a combination of geoid, hydrodynamic models, and tide gauge/GPS buoy. • Thanks for the great job from Randy Herr & NAVO crew